1. Field of the Invention
The present invention relates to a positioning apparatus for incorporation to apparatuses such as semiconductor exposure apparatuses, precision measuring apparatuses, precision processing apparatuses, and the like, requiring highly accurate processing and measurement. The present invention also relates to an exposure apparatus having the positioning apparatus and to a device manufacturing method using the exposure apparatus.
2. Description of the Related Art
It is necessary for semiconductor exposure apparatuses, precision measuring apparatuses, and precision processing apparatuses to be capable of highly accurately positioning a movable table. In such apparatuses, a laser interferometer is widely used for position measurement. However, laser interferometers are disadvantageous in that they are susceptible to fluctuations, i.e., changes in the refractive index caused by changes in temperature, humidity, and pressure of air along a length-measuring optical path. Therefore, a typical precision apparatus requires a constant-temperature atmosphere for maintaining the temperatures of apparatus components at a constant level. The atmosphere is maintained by supplying the apparatus with air maintained at a constant temperature. However, air currents passing over many heat-radiating elements, such as motors, included in the apparatus change their courses in a complicated manner to be mixed with air warmed by the heat-radiating elements. This continuously changes the temperature distribution along the length-measuring optical path and makes it difficult to maintain the temperature at a constant level.
In addition, if the movable table is provided with heat-radiating elements extending therefrom, the heat-radiating elements would exist near the optical path. Moreover, when the movable table moves, the elements also move while changing their forms. This creates a complicated flow field around the elements, thereby considerably increasing temperature fluctuations along the optical path. Hence, the laser interferometer often becomes affected by measurement noise due to such temperature fluctuations. Consequently, accuracy in position measurement becomes degraded. In order to prevent such degradation of measurement accuracy due to temperature fluctuations along the optical path, a first example of the related art discloses a method in which the optical path is provided with a cover that is extendable/contractible in the optical-axis direction or is sandwiched by a pair of parallel plates so as to block the entry of external air into the optical path and stop air currents inside the cover.
A second example of the related art discloses another method for preventing degradation of measurement accuracy in which the refractive index of air along the optical path is stabilized by supplying a laminar flow inside the cover provided over the optical path. Further, a third example of the related art discloses another method in which the refractive index of air along the optical path is stabilized by providing a baffle for suitably directing air currents near to the optical path so that air with a constant temperature and humidity is directly and locally supplied to the optical path. Furthermore, Japanese Patent No. 3089802 discloses a stage-position-measuring apparatus in which influence of the above fluctuations in measuring the position of a movable stage with a laser interferometer is reduced by providing a cover for each of a reference-beam path and a measuring-beam path of the laser interferometer and by supplying temperature-controlled air to the inside of the cover.
The first and second examples of the related art, however, have problems that the optical-path cover limits stage movement and that stage movement allows entry of ambient air to the inside of the cover, thereby changing the refractive index of air therein. In the third example of the related art, if apparatus components are provided near the optical path or if a blowhole is provided far from the optical path, the flow rate of air may vary with measured location and it may be difficult to supply air having a uniform refractive index to the entirety of the optical path. Further, since turbulence may occur around the optical path and ambient air thereof may be drawn into the turbulence, it is highly possible that the refractive index will fluctuate along the optical path. It is also disadvantageous that, in order to supply air having a highly controlled constant temperature and humidity, a high-cost large-scale apparatus for controlling and supplying air is required.
Furthermore, if the stage-driving mechanism is constituted by a flat linear motor or the like, the stage needs to be mounted by using mounting cables that would extend therefrom. Mounting cables radiate heat. That is, heat-radiating elements would exist in close proximity to the length-measuring optical path. Therefore, in the above three examples of the related art, it is difficult to fully minimize influence of the heat-radiating elements provided near the optical path. Additionally, in such a stage-driving mechanism, the mounting cables move in response to stage movement while changing their forms. As the cables move fast, ambient air heated not only by heat radiation from the movable table and the cables but also by heat radiation due to electrical overcurrent between the movable table and a base is complicatedly stirred up by the cable movement. When the heated air reaches the optical path of the interferometer, temperature along the optical path may fluctuate. This also shows that it is difficult to reduce temperature fluctuations along the optical path of the interferometer by applying only the method of any of the first to third examples of the related art.